High alloyed, stainless, austenitic steels or nickel base alloys containing up to 60% nickel conventionally have been used for objects which during a long period of time are subjected to high temperatures in combination with mechanical loading in oxidizing environments. These alloys usually have a high oxidization resistance and often also a very high creep fracture resistance, but because of the increasingly high demands which are raised upon materials for the present field of use there has arisen a need of materials having still better oxidization resistance in oxidizing environments in combination with very good creep fracture resistance, a combination of features which has not satisfactorily been achieved with presently known alloys.
Another problem with known alloys of the above mentioned kind is that they have a comparatively great tendency to take up carbon and nitrogen when exposed in carburizing atmosphere or in environments which involve a risk for the taking up of nitrogen at high temperatures. This particularily concerns austenitic steels but to an essential degree also nickel base alloys. Also attacks from gaseous halides and metal oxides in certain environments may involve problems.
The above mentioned problems will be particularily accentuated in those cases when the material is subjected alternatingly to carburizing and to oxidizing media at high temperatures, or, which sometimes even may occur, in environments which at the same time may act oxidizing as well as carburizing. Those situations when the material in hot condition is exposed to ambient air after having been subjected to carburizing in an furnace at a high temperature are examples of alternatingly carburizing and oxidizing exposures. Similar conditions may occur in furnaces where it from some reason is difficult to maintain a balanced atmosphere. Further may be mentioned furnace linings which are subjected to coke depositions. It is conventional to remove such depositions by burning them off, wherein air is supplied for the combustion, which is a further example of exposure to alternatingly carburizing and oxidizing media. Finally, treatment of poorly degreased goods in oxidizing atmosphere at high temperatures is an example of a situation where carburizing and oxidizing may occur at the same time.